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The Surface Science of Titanium Dioxide

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The Surface Science of Titanium Dioxide Surface Science Reports 48 (2003) 53±229 The surface science of titanium dioxide Ulrike Diebold* Department of Physics, Tulane University, New Orleans, LA 70118, USA Manuscript received in final form 7 October 2002 Abstract Titanium dioxide is the most investigated single-crystalline system in the surface science of metal oxides, and the literature on rutile (1 1 0), (1 0 0), (0 0 1), and anatase surfaces is reviewed. This paper starts with a summary of the wide varietyof technical ®elds where TiO 2 is of importance. The bulk structure and bulk defects (as far as relevant to the surface properties) are brie¯yreviewed. Rules to predict stable oxide surfaces are exempli®ed on rutile (1 1 0). The surface structure of rutile (1 1 0) is discussed in some detail. Theoreticallypredicted and experimentallydetermined relaxations of surface geometries are compared, and defects (step edge orientations, point and line defects, impurities, surface manifestations of crystallographic shear planesÐCSPs) are discussed, as well as the image contrast in scanning tunneling microscopy(STM). The controversyabout the correct model for the (1 Â 2) reconstruction appears to be settled. Different surface preparation methods, such as reoxidation of reduced crystals, can cause a drastic effect on surface geometries and morphology, and recommendations for preparing different TiO2(1 1 0) surfaces are given. The structure of the TiO2(1 0 0)-(1 Â 1) surface is discussed and the proposed models for the (1 Â 3) reconstruction are criticallyreviewed. Veryrecent results on anatase (1 0 0) and (1 0 1) surfaces are included. The electronic structure of stoichiometric TiO2 surfaces is now well understood. Surface defects can be detected with a varietyof surface spectroscopies. The vibrational structure is dominated bystrong Fuchs±Kliewer phonons, and high-resolution electron energyloss spectra often need to be deconvoluted in order to render useful information about adsorbed molecules. The growth of metals (Li, Na, K, Cs, Ca, Al, Ti, V, Nb, Cr, Mo, Mn, Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au) as well as some metal oxides on TiO2 is reviewed. The tendencyto `wet' the overlayer, the growth morphology,the epitaxial relationship, and the strength of the interfacial oxidation/reduction reaction all follow clear trends across the periodic table, with the reactivity of the overlayer metal towards oxygen being the most decisive factor. Alkali atoms form ordered superstructures at low coverages. Recent progress in understanding the surface structure of metals in the `strong-metal support interaction' (SMSI) state is summarized. Literature is reviewed on the adsorption and reaction of a wide varietyof inorganic molecules (H 2,O2,H2O, CO, CO2,N2,NH3,NOx, sulfur- and halogen-containing molecules, rare gases) as well as organic molecules (carboxylic acids, alcohols, aldehydes and ketones, alkynes, pyridine and its derivates, silanes, methyl halides). * Tel.: 1-504-862-8279; fax: 1-504-862-8702. E-mail address: [email protected] (U. Diebold). 0167-5729/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0167-5729(02)00100-0 54 U. Diebold / Surface Science Reports 48 (2003) 53±229 The application of TiO2-based systems in photo-active devices is discussed, and the results on UHV-based photocatalytic studies are summarized. The review ends with a brief conclusion and outlook of TiO2-based surface science for the future. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Titanium oxide; Scanning tunneling microscopy; Single-crystalline surfaces; Adhesion; Catalysis; Chemisorption; Epitaxy; Growth; Interface states; Photochemistry; Surface relaxation and reconstruction; Surface structure; Morphology; Roughness; Topography Contents 1. Introduction . 57 1.1. Motivation . 57 1.2. Applications of TiO2.................................................... 59 1.3. Outline of this review . 64 2. The structure of TiO2 surfaces . 65 2.1. Bulk structure . 66 2.1.1. Bulk defects . 68 2.2. The structure of the rutile TiO2(1 1 0) surface . 70 2.2.1. The (1Â1) surface . 70 2.2.1.1. Bulk truncation . 70 2.2.1.2. Relaxations . 72 2.2.1.3. Appearance in STM and AFM . 74 2.2.1.4. Surface defects . 78 2.2.1.4.1. Step edges . 78 2.2.1.4.2. Oxygen vacancies created by annealing . 81 2.2.1.4.3. Oxygen vacancies created by other means . 84 2.2.1.4.4. Line defects . 84 2.2.1.4.5. Impurities . 84 2.2.1.4.6. Crystallographic shear planes . 85 2.2.2. Reconstructions . 88 2.2.2.1. Reconstruction under reducing conditions: the structure(s) of the (1Â2) phase . 88 2.2.2.2. Restructuring under oxidizing conditions . 89 2.2.3. Recommendations for surface preparation . 92 2.3. The structure of the rutile (1 0 0) surface . 93 2.3.1. The TiO2(1 0 0)-(1 Â 1) surface . 93 2.3.2. Reconstructions . 95 2.3.2.1. The microfacet model of the rutile TiO2(1 0 0)-(1Â3) surface. 95 2.3.2.2. Is the simple microfacet model valid? . 96 2.4. Rutile (0 0 1). 96 2.5. Vicinal and other rutile surfaces . 99 2.6. Anatase surfaces. 99 2.6.1. Anatase (1 0 1) . 100 2.6.2. Anatase (0 0 1) . 102 2.6.3. Other anatase surfaces . 103 2.7. Conclusion. 103 U. Diebold / Surface Science Reports 48 (2003) 53±229 55 3. Electronic and vibrational structure of TiO2 surfaces. 105 3.1. Stoichiometric TiO2 surfaces . 105 3.2. Reduced TiO2 surfaces . 109 3.2.1. Defect states . 109 3.2.2. Band bending . 110 3.2.3. Identi®cation of the reduction state with spectroscopic techniques. 110 3.3. Vibrational structure . 111 4. Growth of metal and metal oxide overlayers on TiO2 ................................. 112 4.1. Overview and trends . 112 4.1.1. Interfacial reactions . 112 4.1.2. Growth morphology(thermodynamic equilibrium). 115 4.1.3. Growth kinetics, nucleation, and defects. 121 4.1.4. Film structure and epitaxial relationships . 122 4.1.5. Thermal stabilityof metal overlayerson TiO 2-SMSI . 122 4.1.6. Chemisorption properties . 124 4.2. Metals and metal oxides on TiO2 ........................................... 124 4.2.1. Lithium . 124 4.2.2. Sodium . 124 4.2.3. Potassium . 125 4.2.4. Cesium. 126 4.2.5. Calcium . 127 4.2.6. Aluminum . 127 4.2.7. Titanium. 127 4.2.8. Hafnium. 128 4.2.9. Vanadium . 128 4.2.10. Vanadia . 129 4.2.11. Niobium. 130 4.2.12. Chromium . 132 4.2.13. Molybdenum. 132 4.2.14. Molybdena . ..
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